The NIH Animal Genetic Resource (NIHAGR) maintains an array of animal model species and, within species, a large number of invaluable genotypes used in biomedical research. For this reason, establishing an embryo cryobank is essential. Cryopreservation and storage of preimplantation stage embryos in liquid nitrogen offers a convenient and economical method for preserving strains and stocks of rodents and rabbits for indefinite periods of time. In addition to stock maintenance, embryo cryopreservation offers other important benefits. First, a bank of cryopreserved embryos provides insurance against the loss of genetic resources due to disease, breeding failure, natural disaster or inadvertent genetic contamination. Second, unavoidable genetic changes due to genetic drift can be minimized, thereby facilitating the development of "Type Cultures" of defined, inbred mouse strains for periodic restandardization of foundation breeding colonies. Third, the international distribution of breeding nuclei in the form of cryopreserved embryos is simpler, less expensive, more humane and easier than the transport of animals. Finally, embryo collection, cryopreservation and transfer offer effective and inexpensive methods for interrupting the vertical transmission of many murine diseases.